This invention relates to an electric power steering system for generating steering assist force by a motor and in particular to a configuration of an electric circuit of the electric power steering system.
The electric power steering system is an apparatus for generating steering assist force by a motor in response to steering torque occurring in a steering shaft. In recent years, demand for an electric power steering system for a large-size vehicle has grown sharply. When implementing it in the large-size vehicle, the required steering assist force is increased. Therefore, larger electric power must be supplied to the motor. However, to use only a main power supply implemented as a battery, such large electric power may be unable to be sufficiently provided. Then, a configuration is proposed in which an auxiliary power supply of a secondary battery is provided aside from the main power supply and usually only the main power supply is used and when larger electric power is required, the main power supply and the auxiliary power supply are connected in series for supplying electric power (For example, refer to Japanese Patent Laid-Open No. 2005-287222). The auxiliary power supply is charged using the battery when only the main power supply is used.
In the configuration as described above, the auxiliary power supply discharged as it is used needs quick recharging. Since the auxiliary power supply is charged using the battery, if it is charged frequently, the frequent charging becomes a burden on the battery. However, any other method than the method for charging the auxiliary power supply is not yet proposed.